This invention relates to controlling a high frequency link cycloconverter system, and more particularly to a voltage controller and method for operating a cycloconverter to control power flow to and from a high frequency resonant tank circuit.
The high frequency link cycloconverter system originally disclosed in U.S. Pat. No. 3,742,336 to B. D. Bedford and also in allowed application, Ser. No. 419,490 now U.S. Pat. No. 3,882,369 to W. McMurray, both assigned to the same assignee as this invention, can be considered as an inverse-series connection of two component cycloconverters. Variable commutation energy is provided by operating the linking parallel resonant tank circuit above its resonant frequency since the capacitor power increases and the inductor power decreases as the frequency is raised. In a typical application supplying adjustable voltage and frequency to an ac motor or other inductive load, the output cycloconverter is in the active or motoring mode converting power from the high frequency tuned circuit to a low frequency output, and the input cycloconverter is in the regenerative mode converting 60 Hz polyphase voltage to the high frequency tank. During the regenerative operation of the system when power is returned to the source, the role of the component cycloconverters is reversed.
In this cascaded cycloconverter system, the high frequency tank is a relatively small reservoir of energy compared to the power rating of the whole circuit. Therefore, when the load on the output cycloconverter fluctuates the tank voltage will tend to vary over wide limits. Such a condition is detrimental to the operation of the system. To maintain the tank voltage within the prescribed limit, the control circuit of the input cycloconverter should have fast response characteristics and additionally should be insensitive to supply frequency drift, distortion free, and capable of reversing the direction of power flow in the regenerative operation of the system. It is also desirable to have a programmable power factor at the 60 Hz input line. Many of the same control features are needed for the successful operation of a simplified high frequency link system which does not require the output cycloconverter and can be used as a VAR controller for those applications requiring only reactive power control.